This invention relates to a nut/clutch assembly for a power screw, and more particularly to such a nut/clutch assembly for use with a power screw linear actuator.
Generally, linear actuators are mechanical devices that provide a linear thrust or force to move a variety of loads (often referred to as an application) either directly or through or linkages. One type of linear actuator utilizes an electric motor as the power source. Oftentimes, the electric motor output is reduced in speed via a suitable speed reducer gear box or gearhead rigidly secured to the motor. These combination motor/speed reducers are referred to as gearmotors. In certain types of electrically operated gearmotor linear actuators, the output shaft of the speed reducer drives a power screw. A nut threadably engageable by the power screw is coupled to a particular load or application so as to move a portion of the application through a predetermined distance and to apply desired force levels to the application for carrying out a desired function. Of course, linear actuators may produce almost any desired output force or be driven at virtually any speed, depending on the characteristics of the drive motor and the amount of speed reduction achieved by the gearmotor and the pitch or lead of the threads on the power screw. However, many fractional horsepower gearmotors have output speeds varying between about 60 and 200 revolutions per minute (rpm), have linear rates of travel ranging between about 8-65 inches per minute, have output torques up to 65 inch-pounds, and exert thrust (either in compression or tension) loads up to 750 pounds.
In a typical electrically driven linear actuator, such as is commercially available from the assignee of the present invention, Emerson Electric Co., under the registered trademark GEARMASTER.RTM., and under the trade designation A13, a power screw may typically have a synthetic resin (e.g., an acetal) or a bronze nut threaded onto the power screw. Trunnion pins or other means project outwardly from the sides of the nut so as to be coupled to an application so that the nut is forcefully driven in a direction along the rotary axis of the power screw such that a portion of the application will be forceably moved via the trunnion connection between the nut and the application.
Of course, it will be recognized that if the linear actuator continues to operate in one direction or the other, the nut threaded on the power screw will either move inwardly on the power screw until it abuts the gear housing, or until it becomes unthreaded from the free end of the power screw. One solution to control the length of the stroke of the nut along the length of the power screw is to provide a timing gear train driven by the rotor shaft, output shaft, or intermediate countershaft of the gearmotor with the timing gears operating electrical limit switches which allow the electric motor to be energized in one direction for a predetermined number of revolutions, and then to de-energize the motor, or to automatically reverse its direction of rotation. Such timing gearmotors are also commercially available from the Gearmaster Division of Emerson Electric Co., and from a variety of other manufacturers and suppliers. One such timing gear/limit switch linear actuator is illustrated in the Dayton Electric Manufacturing Co., of Chicago, Ill., installation and instruction manual for its models 4Z845 and 4Z846 linear actuators, which is included within the file wrapper of the present invention, and which is best shown on page five thereof.
While these timing gears/limit switch controls have worked well for their intended purposes, it will be appreciated that the necessity of a gear train, together with one or more microswitches for controlling operation of the gearmotor, is expensive and is relatively complicated. Also, in order to vary the length of the stroke of the nut on the linear actuator, it is necessary to change the gear ratio of the timing switch or to adjust the relative location between the actuating cam of the gear train and the limit switch. This may be done in the field, and may require the gearmotor to be at least partially disassembled (i.e., a cover must be removed from the timing gear train). Because of the requirement of such disassembly to adjust the microswitches, there is always a possibility that the gearmotor will not be properly assembled. Further, these timing gear trains and limit switches do not positively stop movement of the nut after a predetermined stroke, but rather de-energize the motor after a predetermined number of revolutions. In certain limit switch gearmotors, supplemental mechanical means are provided to positively stop the nut in the event the limit switches fail. These supplemental mechanical stop means may cause damage to the linear actuator or to its application if they are ever required to be used in that they may join the nut on the power screw or apply impaact loads to the application. In certain instances, it may be found that the actuation of the limit switch tends to drift relative to their adjusted position such that the length of the stroke may also vary over time.
As shown in U.S. Pat. No. 3,704,765, an overload clutch for a gearmotor linear actuator is disclosed. The clutch includes a first component connected to the nut which travels through a tubular sleeve. A second component extends through the tubular sleeve for the non-rotatable attachment thereof to a load. Belleville spring washers are arranged in face-to-face relation on the second component between the first and second component, the nut, and the first component so as to apply rotational restraint of the load to the traveling nut while allowing frictional resistance relative to rotation upon the traveling nut when the applied torque exceeds the frictional resistance of the compressed belleville spring washers. While this overload clutch may have worked well for its intended purposes, it did not positively limit the stroke of the linear actuator, but rather caused slippage or de-clutching action upon the applied torque to the linear actuator exceeding a predetermined amount. Thus, the stroke may vary with corresponding changes in the torque limiting characteristics of the clutch, or the torque characteristics of the application.
Reference may also be made to U.S. Pat. Nos. 3,559,500 and 3,587,796 showing linear actuators in the same general field as the present invention.